Successful rejection for breach of the condition as to satisfactory
quality, by reference to the Sale of Goods Act, would appear to be
extremely unlikely. Although allowing the seller a chance to repair
does not in itself amount to acceptance, it's almost certain that using
the car for 2 1/2 years and 50k miles would amount to acceptance.
However, if the car is on Hire Purchase or Conditional Sale, the
situation may be different, as the rules on acceptance don't apply.
Instead, you would be looking at whether the contract had been affirmed
in the knowledge of the fault(s). Termination of an HP contract for
breach on the supplier's part is in theory possible for a very long
time after the contract is made -- although the remedy would not
necessarily include a full refund.